There are several reasons why the Crusades are considered to be so different from other wars we are more familiar with today.
For example, the Crusades were less a political affair than a religious one. Crusaders were interested in fighting in these wars because they believed these to be a mandate from God. The rewards that they were promised were not as concrete as gold or land. Instead, they were promised salvation.
Another important characteristic of the Crusades was the fact that the Church, which we do not usually associate with wars, was behind them. In fact, the Catholic Church encouraged believers to fight in these wars.
Finally, another important difference between the Crusades and more modern wars is the impact that they had in society. The effect of the Crusades deeply shaped Medieval society and culture in Europe, from literature, to music, to social values. Few, if any, modern wars have had such a deep impact in society.